Biomechanics of plant growth

Abstract

Growth of turgid cells, defined as an irreversible increase in cell volume and surface area, can be regarded as a physical process governed by the mechanical properties of the cell wall and the osmotic properties of the protoplast. Irreversible cell expansion is produced by creating a driving force for water uptake by decreasing the turgor through stress relaxation in the cell wall. This mechano-hydraulic process thus depends on and can be controlled by the mechanical properties of the wall, which in turn are subject to modification by wall loosening and wall stiffening reactions. The biochemical mechanisms of these changes in mechanical wall properties and their regulation by internal signals (e.g., hormones) or external signals (e.g., light, drought stress) are at present incompletely understood and subject to intensive research. These signals act on walls that have the properties of composite materials in which the molecular structure and spatial organization of polymers rather than the distribution of mechanical stresses dictate the allometry of cell and organ growth and thus cell and organ shape. The significance of cell wall architecture for allometric growth can be demonstrated by disturbing the oriented deposition of wall polymers with microtubule-interfering drugs such as colchicine. Elongating organs (e.g., cylindrical stems or coleoptiles) composed of different tissues with different mechanical properties exhibit longitudinal tissue tensions resulting in the transfer of wall stress from inner to peripheral cell layers that adopt control over organ growth. For physically analyzing the growth process leading to seed germination, the same mechanical and hydraulic parameters as in normal growth are principally appropriate. However, for covering the influences of the tissues that restrain embryo expansion (seed coat, endosperm), an additional force and a water permeability term must be considered.

A finite strain fibre-reinforced viscoelasto-viscoplastic model of plant cell wall growth

“…Two hypotheses, namely turgor driven creep and new material deposition, are widely accepted to be the fundamental aspects of the anisotropic cell wall growth [9] [45]. Based on these hypotheses, it is assumed that the growth of cell walls refers to two independent kinematics, the turgor-driven isochoric expansion and the plastic volumetric deformation.…”

“…wall thickness, as reported extensively in the literature [7][14] [15]. More importantly, the dynamic process of in vivo cell expansive growth depends on a delicate mechanical regulation to maintain the balance of wall-loosening and wall-stiffening processes [9]. A proper mechanical model should adequately represent these constitutive responses which indicate an intrinsic role of mechanics 3 in cell wall expansive growth.…”

“…The irreversible expansion is therefore referred to as plastic deformation. Although the molecular details of wall loosening are largely unknown, there is convincing evidence that cells produce some kind of a wall-loosening factor that mediates a metabolically controlled chemo-rheological process facilitating the plastic deformation of the cell walls under the tensile force produced by turgor pressure [9]. The idea of a wall loosening enzyme, e.g.…”

“…However, because both are time-dependent, it is not always easy to distinguish viscoelastic response from growth. For example, whether in vitro growth is a viscoplastic or viscoelastic response is still an open question which provides a typical case to illustrate the complexity the time-dependent behaviours of the cell wall [6] [9]. In general, the mechanical response of the growing cell wall may contain both the viscoelastic and viscoplastic components.…”

“…Lockhart equation and its other modified expressions (see, e.g. [9] [24][25][26][27][28]) shed light on the viscoplastic behavior of cell wall growth and has been extensively used in biological research.…”